The present invention relates to energy absorbing devices formed by producing a composite of a relatively low density polymeric foam and an energy absorbing visco-elastic polymer, and more particularly to an energy absorbing article formed from a relatively low density foam and having a visco-elastic portion permanently fixed in the area or areas of the article through which the greatest quantities of energy are normally transmitted.
In many energy absorbing applications an article having a relatively large surface area is employed to absorb energy, even though most of the potentially damaging energy is transmittable only through a relatively small portion of that overall area. For example, it is known to use energy absorbent material to provide a full footwear insole inside shoes to reduce transmission of vibration to the foot and leg, yet potentially harmful vibration is generally only transmitted at the heel and at the metatarsal heads. This is also true of other energy absorbent devices such as gloves, (where only limited areas of the fingers and palms are usually involved) chair and seat pads, helmets and similar protective headgear, and the like. Such applications have many similarities, but they are also distinctly different. Truckers, for example, experience sustained vibration to the hands from the steering wheel, to the foot from the fuel pedal, and to the spine from the cab/seat vibration. All of these vibrations are different, but all are damaging. Still further applications include facia boards and protection areas of motor vehicles such as arm rests, head rests, dash-board tops and the like. It will, therefore, be understood that while much of the explanation and illustration which follows is directed to insole applications, the present invention is in no way limited to insoles or to footwear.
While it is common to refer to these applications as "energy absorbing," they are in fact more correctly "vibration damping" in many cases. This is particularly true in almost all cases of the most severe types. Thus, the absorption or softening of a single impact can often be satisfactorily accomplished using foams or the like. When there is a continuing series of impacts, however, there is a continuing series of energy transmissions and these become basically vibratory in nature. As such, they are not really effectively absorbed by materials such as foams and, therefore, must in effect be dampened.
Foam type energy absorbing footwear components are of course well known, but they suffer well known deficiencies. When made of foam the insoles have a high compression set which renders the device ineffective, often in a relatively short period. Often also if the foam is of an open cell structure it can provide an ideal breeding ground for bacteria and fungi. Some dense materials such as polyurethane and styrene butadiene rubber have also been used and some are even known to have some visco-elastic properties. However, they generally have a density greater than unity often at least 1.2.
Also, it is often necessary to tailor the visco-elastic properties of this device to the end use. In footwear applications, for example, if the desired effect is to be obtained it is essential to achieve recovery before the next step. While this is usually less than 0.7 sec., in fact, optimum recovery time is different for walking applications (about 600 to 1000 milliseconds), joggers (about 100 to 600 milliseconds), and runners (about 15 to 40 miliseconds). In any case it must be remembered that damping is generally non-linear, and that there must be some delay between recovery and the next step, otherwise there may be resonance.
My earlier patent applications have disclosed and claimed certain novel polyurethene compositions which are "visco-elastic," in nature. That is to say that are solid devices, formed from elastomeric polymers, which act to dampen the transmission of energy in much the same manner as if they contained a viscous liquid. First they distort easily and so distribute the load over the maximum area--like catching a ball in the palm of the hand rather than the tip of a finger; then on compression they become progressively stiffer like a spring, except that unlike a spring they pause for a fraction before recovering, which eliminates resonance and may facilitate a frequency change.
Extremely effective devices have been formed from visco-elastic polymers, but they have one potentially serious disadvantage in applications where weight is at a premium. They have a relatively high density. The density of formed visco-elastic devices is generally in the range of from about 0.8 to about 2.0 and is typically about 1.34. It will be apparent that in appliations such as insoles for athletic and running shoes where visco-elastic polymers offer particularly advantageous performance characteristics, their weight is often as much as twice that of the foam insole they generally replace.
Attempts to produce a composite insole having lightweight overall foam construction; with visco-elastic elements at the heel and/or the ball of the foot, have proven singularly unsuccessful. Similar lack of success has been encountered in attempts to produce components of visco-elastic elements and leather, felt, or other overall insole substitutes. Mechanical or chemical adhesives sometimes provide seemingly adequate initial adhesion, but after a very brief period of use, the visco-elastic portion either separates from the main insole structure, or is displaced in such a way that it is rendered inefficient, if not effectively inoperative.
One purpose of the present invention is to provide a composite energy absorbing device having an average density less than about 0.5, having visco-elastic inserts permanently affixed on those areas of the device which are subject to particularly severe or resonant vibratory energy.
Another purpose of the present invention is to provide a lightweight footwear component having an average density no greater than about 0.5, composed of a foam element with inserts of an energy absorbing visco-elastic polymer molded in the critical portions of the component, such as the heel, the metatarsal heads and/or the longitudinal arch.
A further purpose is to provide a means whereby in use such component assists aeration of the foot, and is formed from materials which do not harbor or encourage the growth of bacteria and fungi.